Another possibility:
Back when Uninterruptable Power supplies were expensive, I used to repair and refurbish them myself. I have a lot of expensive gear (My projection TV, for example, had bulbs that might blow if the power went out, leaving them hot without the fan cool-down cycle - and those bulbs were $300 a pop) The one that ended up in my bedroom let out a chirp most mornings before 6 a.m. It normally only emitted such chirps for voltage spikes (voltage drop-outss made a different sound) Since it was a UPS, I knew it wasn’t tapped - or plugged into the phone line, and I decided it was the power company switching in a local generator, especially since it occurs at a different time in the winter vs. summer.
A brief glitch in the power could affect a cheap or sensitive phone.
ON ROOM CONVERSATIONS:
There is a device known as an infinity transmitter that will send conversations in a room over the phone line. Basically, the bugger calls your bugged phone and sends a signal that blocks the ringer, but ‘opens the mike’, turning it into a bug even with the handset on the hook (the hook is a simple switch, easily bypassed)
Sometimes you may hear a brief chirp or partial ring when the bugger calls you, but rarely at the exact same time each day (Yeah, they could put it on a timer, but there are many reasons why this is seldom done.) Nowadays, you can just *69 after a chirp, and if you consistently get connected to your last legitimate incoming call, you can rest easy
Its pretty cheap and easy to build a concealed line-powered room bug that runs on line current. How many wall-wart DC adapters do you have for clocks, answering machines, phones, and other 24/7 devices? Any one of them could have a line powered bug of respectable power. It might take some time and care to install a bug in an existing wall-wart, but it would take mere seconds to replace a wall-wart with a pre-doctored one. Also, it was always possible to install a bug inside an outlet - self-powering, potentially long range and very unlikely to ever be discovered. How often do you replace outlets?
My point? Either get an expensive sweep [not my suggestion, unless you have really strong concerns] or chill out. Sorry but that’s pretty much the choice. It stinks, but that’s how it is.
Be comforted, however. There are few things as boring as monitoring a room bug, transcribing the tape from a voice-activated unit or reading the transcripts. Your hubby’s ex may derive sick pleasure from bugging you, but trust me, you’ve already exacted your revenge by burning up the precious hours of the Ex/PI’s life by subjecting them to the stunningly banal conversations of someone else’s daily life - or worse making some slob transcribe them.