Dear Whoever Made a Wall of Balloons in the Entry of My Cubicle:
You, sir/madam, are awesome. My day has been made. No one has really ever made a big deal for my birthday before. And the best part is, you know me well enough to know that today's my last day in the office this week, so since I'll be on business travel on my real bday (Friday), you've helped me celebrate in advance while I'm in town!
So 'fess up to this so I can thank you! I'm sure I have already tried asking you if you did this and you (like everyone else) has denied it. I have two people on my list of suspects of who you might be, but so far nothing conclusive. Just tell me so I can be appropriately grateful!
PS Thanks for giving me a great excuse for having done no work this morning, despite being in the office for over an hour. You can't work if you can't get to your desk!
(PPS On the off chance you are the Unattainable Coworker, I really do appreciate it. I know why you'd keep your mouth shut while we're at work [you don't want to start more rumors if people ask me who did this and I tell them it's you], but tell me in private. I promise I'll keep it a secret. I promise I won't even take it as a hint that you've changed your mind. I know you've made your mind up and that's life, but if you did this, thank you for showing me you still care.)