I know of one case where an individual claimed to have a muscular-skeletal condition and was off long term sick. He met with a manager about how best to support him back to work by making reasonable adjustments / light duties. However, the individual maintained that they were in constant pain and couldn't walk further than 10 metres without needing to sit down.
Unfortunately for him, this individual was a children's football coach in his spare time which the company knew nothing about. That very night, there was a football tournament and this individual was filmed running up and down the pitch, heading the ball, doing keepy-ups, the works. The person who filmed him was the manager's line manager from that morning, he was there with his son playing in an opposing team.
In the end, the "sick" person was dismissed, lost the ET etc.
The difference in the case I suppose is the conversation that you had with your line manager regarding your capability for work. If you made out that your throat was terrible and you could barely leave your bed then suddenly you're filmed standing outside watching a rugby match, then it doesn't look good.
It's not like this was a mandatory caring requirement for your child. They could have missed the match if you weren't fit to take them.
Your colleague blabbing all around is pretty crappy, if you're in a union, get your rep involved. If not, I'm hoping you have a decent colleague who is sharp and can attend any investigation meeting with you.