The system should be pressure less. Positive pressure could escape in p traps, if pressure is "high" enough. Doesnt have to be in bars. If one p trap is low holding water, it can easily bubble up. A ptrap could have a rip and water is lost in it and level too low. See if you can find evidence of faulty ptrap right beneath it. dried up water spots. So you could try to manually fill up the ptraps regular and see if the smell will be gone. Not many in bathroom, you have sink and shower, bath? Still smell? Then maybe toilet. NO more smell then one of the ptraps. Also a negative pressure is possible, but then you should be connected to large drain system. It can suck out water from ptraps and then level is slightly down and lock is gone, ejecteur working. Ptraps should be made with enough water lock. The system should be atmospheric. So is your vent pipe not (partially) blocked? Evaporation of water, but that takes more time, but in combination maybe with a tiny leak? The tiniest ptrap would be in the shower. The lit in the floor, so the most vulnerable. Maybe 3 or less cm high. Of course , the toilet, is it still closed or not? Seal ring or otherwise a wall of plaster inside still good? You dont see and have to remove toilet from place, but if you lift and it has wall of plaster, you do need to make it new again. Thats the old way of mounting toilets. Or you have to see if you can make connection new way with rubber seal rings. My house here has system like shown on drawing, but in very very rare situations like heavy rain, do have smell and so level of ptrap too low. If I leave house for months, I will close up the drains. Even put some vegetable oil in toilets, at least no evaporation of water in time. Dont like to be surprised, coming home, and smell is coming to me. Welcome home.
