I was in spectator mode on an arena_sawmill match earlier tonight, and I wish I would’ve gotten a screenshot of this incident, but I think I can adequetely describe what I saw:
The action was around the control point, after the 60 second timer had expired. This means the control point is active and can be taken. There was a RED Engineer nearby building a small nest and a BLU Spy, disguised as a RED Pyro, running around on the control point. The RED Engineer never took a shot at the BLU Spy, and was actually backstabbed about 30 seconds later by the same Spy. A little observation on the part of the RED Engineer would’ve given away the Spy’s presence – the control point indicator never changed.
On control point (cp), territory control (tc) and arena matches, there are control points that one team can capture by standing on this point. Each player is worth 1 point (except Scouts, who are worth 2), and the more points a team has on a control point, the faster it is captured. The same principle applies to payload (pl) and payload race (plr) match types, just replace the control point with a moving cart.
The only people who cannot capture points are Spies disguised as the enemy. So this BLU Spy disguised as a RED Pyro is unable to capture the point. Thus, the control point icon remained grey and unmoving. If it were a real RED Pyro on that point, it would change to say “1x” and would begin to be captured by RED. All the RED Engineer needed to do was glance at the control point icon and notice that this “RED Pyro” is not activating the control point – instant giveaway.
Of course, this doesn’t always work. If there are 5 teammates running around a control point, occasionally stepping on and off as they check the area around them, then it is very hard to notice that 1 of them is not properly activating it. On the other hand, if there is just one player, it’s really easy to spot.
This kind of indicators are subtle clues for an observant spy checker.