Dear Namrata,
There are two issues involved in your post. One emerges when the lines of your post are read, and another emerges by reading between the lines.
First Suggestion: Reading the Lines
Tell the security guard to give his report, and based on his report, issue the show cause notice to the team leader. Depending on the explanation, you may take appropriate disciplinary action. If a few others also give a statement, then a warning letter may be given directly.
Second Suggestion: Reading Between the Lines
The incident has happened coram populo. In that case, why did the security guard feel it was inappropriate, and why did no staff member feel the same? Why did they play coy? Did the incident happen in front of you? Did the incident happen in front of other senior staff members? Why did everybody keep quiet?
If the incident happened in front of other staff members and they kept quiet, then it shows that your company has a culture of submissiveness. Beneath the facade of normalcy lies a fear of conflict. Not sitting on the Director's chair is plain common sense. Is it that a culture of informality, to which IT companies are often obsessed, has taken a toll on the sanctity of the hierarchical position?
While you may take disciplinary action against the errant employee, the security guard should also be given a pat on the back for rising to the occasion.
Thanks,
Dinesh Divekar