Where is the tandem breaker located?
To answer the question stated: slot 3 has the tandem (better called "double-stuff" to borrow a term from Harper on this site) breaker in it.

The problem you have is that whoever installed it wasn't paying attention, and installed the tandem in a slot (3) where tandems aren't allowed as per the wiring diagram, which violates the panel's listing.  Swapping the regular breaker in 5/6 with the tandem in 3 will fix this rather easily, though.

(What the diagram says is that slots 1-4 cannot be used for tandem or quadruplex i.e. "double stuff" breakers, while the rest of the slots can, although to install tandems on the right side of the panel in the tandem-capable slots you need to use quadruplex breakers due to the 240V circuits there.)