I've realized that there are a few skills in FiveCore that reference the stealth rules. Likewise, Five Parsecs From Home assumes you have them.
They were originally in the Tactical Primer supplement, since I figured they would clutter up the main rules too much. However, given that they are really more of a core component, I've decided to add them into FiveCore.
Re-download your copy from Wargames Vault, and on the very last page you will find the Stealth rules.
If you already own Five Men in Normandy or the Tactical Primer, they're the same, so nothing to worry out.
To compensate people who did buy the Primer, I'll add another freebie to that supplement this week.