Some background info:

- Our first house (new builder home)

- 3 year old home (we are original owners) with humidity control(both positive and negative)

- 25-35%RH in winter

- 40-50%RH in summer

- Master bath has ceiling fan that is used every time.

- Shower is ceramic tile walls/ceiling/floor with polymer grout.

- Floor and seat of shower are wiped dry after each shower for 3 years now.

- Grout is cleaned every 1-2 months with a brush to removed pink residue buildup (mold or mildue?)

Problem: I have noticed two black mold spots (about 2" square each) forming in the very centre of two tiles on the ceiling. The black mold has not appeared on the grout at any time. However this got me wondering about the possibility of black mold behind the tiles. The grout was never sealed (our first house and we didn't realize it) and is porous so I assume water can easily get behind the tiles.

Should I be worried here? What action if any should I take? Is the absence of grout sealer for 3 years a guarantee that we may have a mold issue behind the tiles?