A roll of 19 works
The tiefling notes that yes, its seems to be a kind of trap door, with a lock that could probably be opened...but he also notices:
The faint outline of an arcane glyph pulsing within the Devourer's mouth.
And in fact (with the 30 on the check), he may also notice a glyph in (at least) one of the other mouths as well.
Now, it may be possible for Lucian to somehow remove or at least temporarily disable the glyph. Assuming he needed to. Maybe.