Learn Finnish: Pomon avainnippu lienee eteisessä lipastolla. - The boss's bunch of keys is probably in the hallway on the dresser.

The verb olla (to be) has a highly irregular potential mood. Instead of adding the potential marker -ne- to its normal stem, it uses a completely different historical root: lie-. For the 3rd person singular, this gives us lienee (root lie- + potential marker -ne- + personal ending -e).

Is lienee commonly used in everyday spoken Finnish to say where things are?

Not really. The potential mood is quite formal and is mostly found in written Finnish, news broadcasts, or literature. In a casual conversation, a Finn would be much more likely to express this by using the regular verb on along with an adverb like varmaan (probably) or ehkä (maybe), saying Pomon avainnippu on varmaan eteisessä...

Why does the sentence use two different location endings for eteisessä and lipastolla?

This highlights the difference between internal and external locative cases. The keys are physically inside the room (eteinen becomes eteisessä, meaning "in the hallway"), but they are resting on top of the furniture's surface (lipasto becomes lipastolla, meaning "on the dresser"). Finnish naturally stacks these case endings in a row, zooming in from the general room to the specific surface.

In the compound word , is the word in the genitive case to mean "of keys"?