What is a common sign of a bed bug infestation?

A common sign of a bed bug infestation is an obnoxious sweet smell, often described as similar to the scent of berries or coriander. This odor is produced by bed bugs and can become more pronounced when the infestation is significant or when the bugs are disturbed. The sweet smell is typically a result of the aggregation of their pheromones, which serve as a form of communication among the insects.

The other options do not indicate a bed bug infestation. Increased ant activity might suggest an ant problem rather than bed bugs, while an odor of pine is not associated with bed bugs at all. Visible webs are characteristic of spider infestations, not bed bugs. Therefore, the presence of a sweet odor is a definitive signal pointing towards a bed bug infestation, making it the correct answer.