I think a key thing as spy is to avoid groups completely. If you reduce the chances to bump into someone then it will make life easier.
I think spies work best when they are trying to pick off the people at the back of the lines, whether it's people running from spawn, or engineers or people going back for health. Kill, cloak, hide.
If you're in a situation where, when cloaked, you're trying to get through a group of enemies, either following, or running through the front line to get behind the enemy, then I highly recommend jumping from side to side. Being able to reduce the area of possible contact with an enemy is the key.
When disguised there is a little bit of timing involved, as well as a clear plan. You need to look like you have a definite movement, nothing gives away a spy as someone just running around after another person. If you attack when they are attacking then it's easier to blend in and not look suspicious.
I do my attacks as spy in a forward motion. Cloak, get in behind the lines, disguise, decloak, run forward with the enemy attackers and attack. No matter the result, I push through to my team-mates, heal and resupply. Rinse and repeat. Obviously if your using the cloak and dagger or dead-ringer then it's different.
There is a huge slice of luck involved with tracking and pushing through as a spy as people are a little unpredictable and sometimes you just get unlucky.
The HP Boost does not stack with the medic-gun.
It heals for 60 HP, and bumps the max HP by 50. At the end of the 30 seconds, the extra 50 HP disappear immediately. If you are above 300 HP at the time, you are reduced to 300 HP (assuming no over-heal). Otherwise, there is no change for your current HP.
If you are currently buffed to 350 by the bar, a health-pack or resupply cabinet will heal you up to 350 HP. However, it appears that dispensers will only heal you back to the original 300.
There is more detail at the official wiki.
Best Answer
If you received an item that didn't appear in your backpack until later this is not unheard of. Usually it's a glitch or a synching error, but the item will usually be held for you and added to your backpack when it can. Restarting TF2 sometimes will help with this.