Xcode doesn’t show up in spotlight results, neither search results

Try rebuild index

From the Apple () menu, choose System Preferences.
Click Spotlight.
Click the Privacy tab.
Drag a folder or an entire volume (your hard drive) to the list.
If prompted for confirmation, click OK.
Remove the item or volume you just added to the list by clicking
it and then clicking the minus ("-") button.
Close Spotlight preferences.
Spotlight will re-index the contents of the folder or volume.

You can also use the command line to rebuild Spotlight’s index.
Open the Terminal application and enter the following command:

sudo mdutil -E /

Authenticate with your administrator password,
and a message appears indicating that the index will be rebuilt.
Spotlight rebuilds the index of your hard disk.
You can track the rebuilding progress by clicking the Spotlight menu.

The Spotlight window does not include files treated as system files by default. Try setting "System files" to "are included":

Or use mdfind:

$ mdfind -name tweetbot
/Applications/Tweetbot.app
/Users/lauri/Library/Application Support/CrashReporter/Tweetbot_4C49445B-E14E-51E5-87DC-7CE59AE6A841.plist
/Users/lauri/Library/Containers/com.tapbots.TweetbotMac/Data/Library/Application Support/CrashReporter/Tweetbot_4C49445B-E14E-51E5-87DC-7CE59AE6A841.plist
/Users/lauri/Library/Containers/com.tapbots.TweetbotMac/Data/Library/Application Support/Tweetbot
/Users/lauri/Library/Containers/com.tapbots.TweetbotMac
/Users/lauri/Library/Speech/Speakable Items/Application Speakable Items/Tweetbot
/Users/lauri/Library/SyncedPreferences/com.tapbots.TweetbotMac.plist