Yeah, what the others said. At the end of a 5 km I have never been able to get more than 95% so in the absence of any other information you could go by what you have seen in a 5 km and add a few beats rather than a formula that may be true for populations but not any one person. Kinda like you would never by a pair of size 8 trainers and hope they would fit on the basis that the average is say size 8 +-2.
The problem was not that you were not carrying an ID but that the other person wasn't, and helpful friends erred on the safe side and reported that it could be you. Carrying a tag would not stop this false positive, it would only make sure you are identified in case of a true positive.
To cut down on false positives you could only post to fb when the track is finished, and anybody who clicked on the link could see you were in fact finished. Or you could use a device or app that has so called Live Tracking. Or you could just not post on fb and instead use a device or app that has a Beacon feature that gives a trusted person a link or sms to find out where you are. For really unknown or dangerous territory you could carry a locator such as a SPOT beacon.
As a general principle I would follow what the paid professionals say, rather than n=1 personal reports, but I googled this for you and this exact point is answered down the text in this first hit: http://www.runnersworld.com/newswire/recreational-marathon-training-good-for-runners-hearts