What is "Test String Do Not Respond" flashing in IM before person's name shows.

[AffectionateOne Melody]

I think my account/IM's are being compromised.

[Rolig Loon]

That's very unlikely, since there is no easy way for anyone outside of Linden Lab to peek into your IMs.  However, if I were a Linden Lab tech trying to repair or upgrade the system, I would send test messages like that.  If you are truly concerned, submit a support case to ask.  Include a screen shot, if you can.  If you are a Premium member, you can get a quicker response by asking Live Chat.