Not far, a few miles likely for and ELT on the ground. Could have been in a passing airplane if you heard it for a long time, or it was close and activated for the 20 mins you heard. If at an airport with a controller of any sort they would have alerted people on the field about it to track it down.Thats the first thing that came to mind when i heard it. Wonder how far away you can pick up the signal on the ground ??
I have seen a half dozen times or so in 25+ years in Aviation that one got set off. Even high winds and and aircraft that is not properly tied down can do it........but during my time working on aircraft I had never seen one accidentally activated.......I would think any signal like this on this frequency would immediately set the search & rescue teams into motion...
121.5 is mainly defunct now and not monitored by satellite any more so it is unlikely that any SAR effort would be launched soley on the basis of an obsolete ELT signal.
I assume that since i had the signal for 20 minutes, It was either in someones garage and accidently activated or it was on a aircraft flying over and went off for some unknown reason.
I think the airport is to far away to pick up the signal.
The 406 MHZ, Can it be monitored by radio or is it mainly satelite ? And if it can be monitored is it 406.000 or other ? I havent seen anything to indicate. .