This could come back to bite you, especially now the relevant tax authorities talk to each other a bit more than they used to.

The correct (and legal) procedure would be to declare your overseas rental income against which you can claim your mortgage payments. If you own the property outright then you are liable to the wealth tax on the value. You can claim the costs of any maintenance etc.

The so-called expat tax allowance is for those living and working in Switzerland who must necessarily maintain a property overseas - for example, it is still lived in by the family or you are a weekly commuter. If you are unlucky, you may be asked to prove this.

If you rent it out then you are not eligible for this allowance, instead it is a source of income.