Please explain this weird response to adb command 127|root@android:/ # find / -iname "aaengine"                                  
find: /proc/19619: No such file or directory


I dont know why I got told about some file I never known about when trying to find aaengine file
Quite easy: find / starts exploring at the file system root (i.e. the very top). So it comes to places you have no access to as normal user, and runs into permission issues. It sees a directory called /proc/19619 and, knowing this is a directory, wants to traverse into it. But it is not permitted to (most likely the permission of that directory is something like drwxr--r--, so everybody can see ("r"ead) it – but only its owner can change into (eXecute)). So your find sees it, but can not dive into it.

As you've been running this command as root, permission problems shouldn't be the issue. But taking a look at the directory in question pops up a different answer: /proc is storing process information. Below this top-level dir, directories are created using the PID of running processes. So /proc/19619 stores information on process 19619. Most likely your find was seeing that directory, and at the same moment that process ended; so when find finally wanted to dive into, it was gone: No such file or directory (not anymore).