Its possible that the drain/toilet seal has failed and sewer gas is entering the house.
This is between the water trap and the sewer, at the connection of the fitting to the pipe.
As for not losing another night's sleep over this, your night's sleep is precisely what you should be worried about.
Until you get the smell investigated, make sure you ventilate the space properly to prevent a build up of sewer gas, which can be flammable/toxic.
Under no account allow the gas to circulate into the living spaces or the bedrooms, where it could cause respiration problems and lead to serious consequences.
ONQ
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All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.