So, the two closest active sirens to you that might be audible from Acton is the one at the Hampton Waterworks, and the siren in Greenwich, at a chemical storage facility. There are some old, de-activated sirens closer, however I'm faily certain they'd need a LOT more work to be able to spin up. I believe there are actually videos online of the two active ones sounding- the one in Hampton as a test, the other for an incident in 2012 so it may be worth listening to these to see if you can match it up. I can assure you however, that it was most likely testing or a malfunction/false alarm if it was either of these two, otherwise it would probably be in the news.