Advice: Tax Return - Condo Rental Income, Non-resident

I've been told that it is not usual to pay house and land tax in the case of privately rented (ie. not registered leases) properties.

1) If this is correct (?), I assume people don't usually file for personal income tax either?

2) What's the risk that the taxman will slap you? Has anyone ever heard about that happening?

3) If rental income tax is levied, would interest on a (thai mortgage) loan from BKK Bank S'pore be deductible?

assumptions:

Non Thai resident, with no other income in Thailand.

Condo in BKK rented out privately, no agent, long term, paid in THB to Thai Bank Account (tenant does not deduct witholding tax). Contractual lease for actual structure is one third of what tenant pays (furniture and services paid separately).

Thank you in advance for sharing your experience.