Just from the context of the sentence, it sounds like a group of people left their hotel to have a picnic, but then realized they left all the stuff for the picnic back at the hotel (a rather specific scenario). The use of the word "picnic" here is somewhat awkward to me. I would say something like "they left the picnic basket" or "they left all the stuff for the picnic."

Picnic = the lunch they had planned to eat during their walk outdoors.

It means that they had planed to have some picnic together, but realized that the food for the picnic was left at the hotel.

"The picnic" here means the basket or other container in which the food and utensils for the outdoor meal (also the picnic) are carried.

The picnic here means the food they had intended to take with them to eat outside, for example in a park or in the countryside. It sounds fine here to me. If it was me who had realised I’d left the picnic back at the hotel, I’d say “Oh no, I forgot the picnic!” (Better English- “I have forgotten to bring the picnic” but probably wouldn’t say that in speech) or “Oh no, I’ve left the picnic at the hotel!”.