can someone connect to my bluetooth without me knowing?

Let’s look at attack possibilities: When using Bluetooth headphones, I can think of the following attacks you might be concerned about:

Traffic snooping (someone reading the data which is going over the connection, or just noticing that a connection exists at all - perhaps revealing that you're listening to something at that time)

Traffic modification (someone altering the data which is going over the connection)

Service disruption (preventing your headphones from working, probably by flooding the airwaves with random or spammy data)

Infiltration (using your PC or iPad's bluetooth capabilities to gain unauthorized access to the PC or iPad)