preventing unauthorized user

I take care of a sick relative . i also have home care aides who come and go from home, along with some others. i noticed the other day the modem was in a different postion on my book case . it was clearly moved. i noticed that on the side that was now clearly visible in the position it was moved in, you can clearly see the password that is printed on the side of the modem. to me it looks like someone moved the router a little so they could get a good look at the password , but forgot to move it back.

how can i block or see if someone is using my service?