SQL Server - connecting to wrong instance?

So, I've ... inherited a SQL server environment. With LanSweeper, I see this particular server has two instances installed: sql001AK and sql001M5. LanSweeper tells me they have similar databases, except for UserDB3, which is on the M5 instance but not on the other. I use SSMS to connect with sql001M5 to look into this user db, but I can't find it. When I pull up the properties of this database, I see in the name attribute the name of the other instance! When logged into the actual server, I can clearly see in the SQL Server Config Manager that it's two separate instances, no aliases defined, no weird clustering, all seems fine. But in SSMS, entering the one name connects me to the other... What could explain this behaviour? How do I connect to my M5 instance?