is it possible to be severely autistic yet not being "visibly autistic"? It is common problem for many "mild" (ASD level-1) autistics to be criticized as "you do not look autistic" or some of its variants. Also sometimes seen in social media that moderate autistics (level-2) (claimingly) facing similar issues to disclose their diagnosis. 

By "visible" I mean the publicly considered easily observable disabilities, such as extreme intellectual disabilities, visibly heavily motor nonfunction or totally unable to move, saliva dripping from mouth, complete nonverbality or observably distorted voice and language, disfigurement or birth-defects, inability to hold urine or stool,  etc. Usually the known to be severe (level-3) autistics have at least one of these "publicly perceived as disability" symptoms. 

Incidentally, autism is not itself a visible condition at all, and the intellectual disability is not itself autism, its sometimes a co-morbid condition with autism but it is not identical to autism, rather autism is basically the impairment in social cognition. Also some of the symptoms of autism may be actually visible or easily observable but these are commonly mistaken as intentional inappropriate behaviour or lack of morality or as some mental illness instead of cognitive difficulties or social disablilities. 

Now my question is; Is it possible that someone has level-3 (severe) autism yet on superficial look or talk the disability does not appear? 

Theoretically it seems to be very possible that a severely autistic person may lack all of the publicly believed stereotype on disability, also can lack intellectual disability, moreover can apply sharp observations and observation to mask and fit-in, yet can have tremendous problem in social aspects. In such case a level-3 autistic too may face "you don't look autistic" situations. And probably all the observable symptoms will be imsinterpreted as mood or behaviour issue. 

But does that happen in reality? or all of the severe (level-3) ASD have also some publicly believed stereotype of disability?


  Note: See also Autism "severity" levels 
  
  Screenshot:
What you propose

Is it possible that someone has level-3 (severe) autism yet on superficial look or talk the disability does not appear?

is nonsense by the usual (DSM) definition of autism, which is based on several criteria which are behaviorally defined, i.e. from an observer's standpoint, to start with

a child must have persistent deficits in each of three areas of social communication and interaction (see A.1. through A.3. below) plus at least two of four types of restricted, repetitive behaviors

Nowhere in there is that presupposed to be just &quot;basically the impairment in social cognition&quot; that may not even be manifested.
(Somewhat of an aside here; DSM-5 has tried to be less &quot;atheoretical&quot; than its III/IV predecessors, but in the end the differences in that regard are minor.)

Now if you want a bit of a deep[er] dive here, the DSM-5 acknowledges that some compensatory behaviors exist and should even be considered in assessing the (actual) severity of the diagnosis.
And a bit of digging through recent research finds a controversy in (re)defining (of sorts) &quot;true&quot; autism to mean Theory-of-Mind-deficits instead of [DSM] behavioral-autism... or at least (thus) identifying &quot;camouflaged&quot; or &quot;highly compensated&quot; autism (understood as ToM-deficits). This is quite a complicated discussion because ToM is multidimensional (or at least its measures are--and some correlate better than others with DSM-autism), and so (varied) are compensatory mechanisms for its deficits. The latter are surprisingly used by &quot;normal&quot; people as well, with perhaps only &quot;shallow&quot; compensation strategies specific to autism.
In the realm of personalities, you have prominent researchers on both sides of this dispute, e.g. Happé (responding to) Fombonne's (editorial). (h-index &gt; 100 for both.)
I'm guessing you've read &amp; bought the ToM-deficits view of autism when you say &quot;autism is not itself a visible condition at all&quot; and &quot;is basically the impairment in social cognition&quot;. Frankly if you look at Happé's last piece I linked, she (and her group) are cautious not to take that extreme position, although I'm sure it came off like that in some popsci article (the BBC at least has an article on people discovering their &quot;hidden autism&quot; thanks [or at least in relation] to Happé's research.)
As a final (recommended) reading here, which may help put this in context, some piece from Allen Frances who was quite involved with
the previous editions of the DSM on the conflicting views on mental illness diagnosis standards in general (e.g. is clinical impairment needed) is also relevant here.