Hello!  My current tenants have been hearing a beeping noise for the last month and a half or so.  My first thought was to change the batteries in a fire/smoke alarm, carbon monoxide detectors, etc. - or some other electronic device.  We've removed devices, unplugged appliances, changed filters, I've had plumbers out to see the hot water heater, I've had appliance guys out to check the fridge - they haven't been able to pinpoint where the sound is coming from and everytime I've been there for 1-2 hour chunks at a time to look around for anything I haven't heard it.

UNTIL TODAY.  The beeping is a 4 second beep that appears to be coming from the basement.   The timing is entirely random, sometimes more than 24 hours apart, sometimes every 30-45 minutes. The tenants swear it's on one side near the washer/dryer - both have been unplugged and they've still heard it.  To mine and my wife's ears it sounded like it was on the opposite side near the circuit breaker, furnace, and modem/router.

One of the tenants was very concerned about the possibility of it being dangerous - possibly carbon monoxide, and this morning mentioned that she would be staying in a hotel until it's resolved, so I had the fire department out today to check for carbon monoxide and they didn't find anything and they even told her pretty much what I've been saying this entire time which is - you might need to sit down in the basement until you hear it and move closer and closer to where it's coming from until you can figure it out.  The truth is, I don't think they want to do that, and are expecting me to do it.  Because the beep happens at random intervals or sometimes days apart, this seems unreasonable to me.

As a last ditch effort, I am having our electric, hvacs, and washer and dryer looked at.  All these service calls will have cost me close to $1000.  But, this is probably cheaper than also throwing in a hotel bill on top of it and I do understand their frustration so I'm willing to do that.  The only other thing I can think of is to setup microphones around the basement and see which has the loudest audio wave when I import it into garageband and narrow down the location that way.  

I guess my question is this:  Does anyone have ANY idea of what it could possibly be? OR any advice on how best to handle the situation if I am handling it incorrectly?  

Thank you!