Wireless doorbell rings randomly but in a different chime-why?

[foreverplaid]

My parents have a wireless doorbell for 2 doors. The chime they have plays a tune. However, recently it goes off on its own, but only plays the usual "ding-dong" sound. They thought batteries might be low and put new ones in. This still happens. Sometimes a few times a day, then it might not happen again for at least a week. I can understand interference, but why the different chime that they don't even have on their doorbell? They have been keeping track of times it happens and it is usually around 9 and 11, both am and pm. They are positive no remotes or radios were being used at the time. I went out with my dad today, and when we came back, the bell went off in the different chime when we walked in, right around 11 am. There were no appliances on at the time. House was empty. What could cause the doorbell to ring a different tune? It never randomly plays the tune they have set-which would be easily explained by interference. Why a totally different ring than the one they have set when someone physically rings the bell?