It's pretty clear to me that the one who created an astral signature can find it and erase it without any checks, just spend the complex action while assensing.

Can something else, other than the original caster, erase a signature? Buddy mage on astral overwatch, for instance? RAW says "a magician", so this seems to rule out adept with the astral perception power -- they'd need to be a mystic adept. Seems to also rule out all spirits save for spirits of man, or any other spirit with spellcasting ability.

If someone other than the caster can erase astral signatures, do they always need to get 3 hits on assensing first? Even for the caster's own spirits (assuming they can erase them in the first place)?

Anyone with the assensing skill can do it. At least that is the way I would rule it. Realize spirits can also remove signatures. Yes, you can even remove someone elses signature as well.

Remember only someone using Astral Perception can erase a signature. Therefore, a Mystic Adept who hasn't spent the PP on Astral Perception can not remove his signatures. I don't think it's explicit, but I would say you can not take the Cleansing metamagic power either without having Astral Perception as a prerequisite. EDIT: it is explicit, just noticed it.

You can erase your own or someone else's signatures. The rules don't say you can't.

If a spirit has the Spellcasting power, they can erase signatures just like a magician can (including signatures created by someone else) because they have astral perception (dual natured to be exact).

I would think you would need to roll 3+ on assensing test to be able to see the spell sig in order to erase it.

I misspoke, I was thinking of the wrong thing. Astral perception is correct, I don't know why I said assensing.