Volume indicator disappeared iPhone

So a couple of weeks back I noticed that when I pressed the volume keys on the side of my iPhone 4 (iOS 5.1), the ghosted indicator did not come up on the screen. I checked my settings and all was fine, so I rebooted the phone and that made it come back! Just a glitch I thought, but a day or so later, the same thing happened again! It returned again after a reboot, as before. It next occurred almost a week later but this time, the notifications drop down menu also stopped working! Once again a reboot sorted it out so I followed the usual process of restoring it, and intact I did this twice, and it's been fine for another week, until today!

I'm not going to do a reboot today, I'm going to see if the notifications glitch occurs again.

I'm a pretty high iPhone user, headphones are used almost every day, I dot use a dock as I have AppleTV, I use only genuine Apple products and I've taken pretty good care of it, I got it on the day of release, so it's almost two years old, barely has a mark on it and has no evidence of water damage in either of the ports and I've cleaned the pocket fluff out of each also. I've loaded half of it up with music and apps, so there's a good 12gb of free memory. So this obviously begs the question, WHY???

Any help will be appreciated peeps, thanks in advance.