Hi.
I do not feel it is something you should be worrying about at this time after having read your message.
I would like you to, if you can, try lighting the candle again a few more times, in the same room, spot, and place as you have previously tried in your message.
whether the fire detector goes off these next few times or not, may not even be of too much importance, that is to say you should not worry.
There could be a number pf physical reasons for this- to a malfunction of the detector, or could even be the altar is placed too close to the detector.
Could also be it was something your neighbors may have been doing, if you have neighbors close. Like in an apartment building.
That said, there is no need at this time for worry, simply observe.
The real emergency situation is if there is an actual fire. Could be the neighbors cooking (or anything else) at odd hours.
Late night cooking- it happens.
Be observant and relax for now.
Blessings.