musty smell in the guest bathroom (new building)

Hello dear forum participants,

We have been living in our small new-build house for about a year. From the beginning, I noticed that there is often a musty smell in the guest toilet on the ground floor. I already spoke to the site manager and the plumber about it back then, but only received answers that it obviously could not have anything to do with the installation. They referred me to look for the source of the smell outside (it must be coming in from outside). In the meantime, I have also opened the transfer shaft but could not detect any smell at all. So I suspect the source is in the toilet. The problem is that the smell does not always occur but definitely happens often (so usually every day, sometimes even multiple times, even if it was aired in between and everything smells "fresh"). Can someone tell me where I should look best so that I can specifically ask the builder again? I cannot detect the fluctuating water level in the siphon, which has already been discussed several times in forums. At least not when flushing the adjacent toilet.

Thank you very much!

tm

Are the installations used regularly? Things like that often happen because of a dry siphon. Or because of bad siphons.

What does the overall installation look like? Where is the ventilation of the wastewater system? Far away? Has a pressure equalization valve possibly been installed in the [Gäste-WC]? Do you have a simple way to look behind the [Vorwand]? Do you have a work plan/detailed plan of the house installation ([Sanitär])?