I suggest that you write to the neighbours saying that you understand that they have some concerns about X, ask them can they please let you know if this is so, and if it is the case what exactly are these concerns.
If they respond then you can approach the tenant, softly and say that you have been told that they were playing loud music late on such and such a night, and would they just think of the neighbours. Keep the conversation light, you can agree that the neighbours here generally are frightful old complainers if that keeps the conversation friendly.
Then write to the tenant, thank them for the conversation and for agreeing to be more considerate of the neighbours. No specific details.
Write to the guards, ask them if there is anything you should be aware of.
This way, if the issue does not go away by itself, these things often do, you will be able to show that you have taken steps to resolve matters.
If there is a real problem, the sooner you know about it the better.
Medium term, you need a better tenant.