Board Thread:General Discussion/@comment-28962359-20160825041654/@comment-1361960-20161029195421

Of note is that Undyne doesn't seem to think that you need a monster soul to escape. She just says to talk to Asgore to try and convince him to let you go home. Only three monsters claim you need a monster soul as well: Toriel, Asgore, and Alphys, all three of which were undoubtedly aware of how Asriel got out and might have made the assumption based on that(Going back to RUINSscavenger's theory).

Complicating the issue is the fact that no human who fell into the Underground that we know of made an attempt to cross the barrier on their own; Chara didn't even try, and the other fallen humans died before they could try. Unless there were unrecorded humans who fell in and managed to slip past Asgore and through the barrier before they were caught and killed, we have no concrete evidence either way to work with.