Cryptic photosynthesis planets

Cryptic photosynthesis, Extrasolar planetary oxygen without a surface biological signature

Considers the possibility that the majority of a planet's photosynthetic life may be hidden in rocks or underwater, which would not give a biological signature in the spectrum of the planet. This may be the case on planets with extreme surface conditions.

From what distance can this signature be detected?

Hmmm makes sense on the surface of it ((no pun intended))

However with erosion of said rocks organic material would be slowly released yes? With water it gets alot less likely to tell IMO unless it's something like phytoplankton.