I'd double check that next time you buy a new ThinkPad. I broke the screen on mine recently, and chose to replace it myself after being quoted an eyewatering (nearly cost of laptop) amount to have it repaired. The tech support person told me in no uncertain terms this would totally void my warranty.
If there are no warranty stickers, and you don't make any obvious changes or damage to the machine, you sometimes get away with 'breaking' the warranty. I've had this occur for a laptop that required opening to change the RAM (voiding warranty by all supporting documentation). I restored the original RAM before warranty, and they fixed it all the same.
Maybe it's just for minor changes, simple user upgrades instead of huge replacements like the screen? shrug I've never had to warranty my Thinkpad anyways thanks to them being tanks.
Just worth checking.