Radio Frequency Regulations for continuous low-power single-frequency signal
Of course it depends on where you are, there are FCC standard and EN standard and whatever standard in other countries.
The main denominator would be field strength. If you stay in the relevant EN 61000 emission curve you don't really need to use the ISM bands, it's simply considered a 'tolerable unwanted emission'. I can do a 4MHz capacitive sensor, for example, without issues if it doesn't go over the limit.
If you need more signal however you need to use the ISM band (the textbook example is an RF heater) and probably comply with the relevant standards (like the electroheating ones, in this case). Similar to your case could be a radar level detector for silos, for example. It's actively transmitting with some significant power but it works inside a standard framework (the ISM one)