@chrxlr I would just like to point out that it's not just "anaa" for everything in Cebuano. "anaa" is just one of the existential demonstratives in the language. It is the existential form of the medial. The following are the existential demonstratives in the language:
Proximal (nearer to speaker than to receiver) - adi'a / ari'a / di'a (all are variations of the first).
Medioproximal (near to speaker and receiver) - ani'a / ni'a
Medial (nearer to receiver than to speaker) - ana'a / na'a
Distal (far from both speaker and receiver) - adtu'a / atu'a / tu'a
So here's how it should have been:
Tagalog | Cebuano
na kay | tu'a kang
nasa | <existential demonstrative> sa (depending on relative distance i.e. is it proximal, medioproxmal, medial, or distal)
narito, nandito | ni'a (the locative demonstratives are not necessary)
narine | di'a (again, the locative demonstratives are not necessary)
nariyan, nandiyan | na'a
naroon, nandoon | tu'a
However, as you said, it's informal. Cebuano does use « ana'a », still as an existential, but for locative only (as opposed to « aduna »; with the addition of the indefinite direct marker « ing », « ana'a » becomes « ana'ay » (ing has been contracted), likewise « aduna » becomes « adunay)). In Tagalog, this is approximately the same as "may, but concerns the location of an object i.e. does it exist in this or that location.
As for "na" (or "nan") in Tagalog, it may have the purpose of transforming the locatives (doon, diyaan, etc.) into existential forms. "nasa" could have implied that "na" is an existential particle, thus removing the necessity of relative distance in pertaining to the existense of something.