It could indicate a serious problem, but more likely it means an aircraft has had a heavy landing.
The ELT is a small transmitter that has it's own battery pack and is activated through an inertial switch. It transmits a repeated tone on the emergency frequency and is used in search and rescue efforts. Unfortunately, a really heavy landing can feel a little like a crash to the airplane and can activate the unit. It depends on how sensitive the switch is.
Alternatively, somebody could have accidentally switched the unit on directly, or, in the States at least, you are allowed to test the unit in the first five minutes of every hour.
Aircraft large and small carry these units and airliners typically listen in on the emergency frequency as they may be the first people to hear the signal if it results from a real incident in the middle of nowhere.
There has been at least one rescue effort in the Rockies that started before the aircraft was even known to be lost. A transiting aircraft heard the signal and was able to report it and get everyone rescued promptly.
BTW, they also manufacture a watch now that'll transmit an ELT signal, though it's operator activated only.
Hope this helps.