The creature has no idea you’re present, and has no inherent reason to start Searching for you or making attacks against you. If they are suspicious or wary of the situation they’re in, however, they may start Searching the area around them anyway, just to check that nobody’s sneaking up on them.
While you are Unnoticed by a creature, they are Flat-Footed against your attacks, and your attack rolls against them have Advantage.
If they (for whatever reason) make attacks against the square you are in, they must must Succeed on a DC 10 Flat Check or the ability automatically Critically Misses and has no effect on you. Any area effects that they use that happen to include your space affect you as normal. (Again, though, they shouldn’t be trying to target you in particular; this should only come up if you’re caught in a cross-fire and one or both parties are unaware of your presence.)