I don’t think someone blew up a container, but, as there are (illegal) hand grenades circulating in Sweden, maybe somebody deposited some in such a container. When it was emptied, all participants got into a hurry as they weren’t qualified for handling explosive devices.
Alternatively, they’ve thought beforehand and placed the sign knowing that some may also want to get rid of their grenades. But as they require extra care, it’s probably a bad idea to store them in a container in the foyer of the police station.

My gf once was at a police office in Sweden for filing a report. Near the entrance, there was a big solid metal container and the sign “no grenades” on it. It was to deposit guns without any questions asked.

It seams the tweet was ahead of its time by half a year 7 month.