Your idea of going thru the breakers 1 by 1 is an excellent way to narrow down the source (if it's local). Obviously if you turn the main breaker off and you still have interference, then the source is outside of your walls.

Things that can cause interference.... pretty much anything!
When caps go bad in equipment, the device may seem to still operate fine but the cap was filtering RF noise that is now broadcasting. Also triacs can start to go bad and cause RF noise. Bad electrical connections that are arcing, can cause RF noise.

So =
Your HVAC. Lighting dimmers. Battery backups. Microwave ovens. Ceiling fans. Computers. A/V equipment. Network equipment. Fluorescent lights. LED lights. EV car chargers (including golf cart chargers).
These are all things I have seen create a ton of RF interference before. Some of these things were 'working as intended' and others had issues that was causing the problem (listed above).

Since this is something that just started. And the fact you said you walked the whole house and the interference is everywhere. You'll be looking for a very high energy emitter that has a high duty cycle. Something like your HVAC. Do you have any large chargers plugged in (like for a car, golf cart, electric mower)?
Good possibility this could also becoming from your neighbors.
Do you live near an airport? Marina?

Also, is the interference actually causing an issue with commands not getting thru from the remote? If not, not worth worrying about is it's not an actual problem.