While not approved for the 172, it's hard to operate in the real world without picking up a little non-forecast light rime on occasion. No cause for panic, just monitor the accumulation and leave icing conditions by the most expeditious means. Severe accumulation might be cause to drop below the deck and land on a road or something, but 172's can carry a pretty good load of even clear ice in a crisis, just keep the power and speed up.

I doubt they departed into known or forcast icing.