"Something went really wrong here," Lennarth Ädel at Tempest Security told the Aftonbladet newspaper.
On Tuesday, a cleaner found the woman, who was unauthorized to be in the building, at 6am near a pool of vomit, urine, and faeces.
Ädel theorized that the woman could have gained access thanks to a technical problem with a door, or through someone letting her into the building.
The head of the Riksdag security team, John Arentoft, figured it may even have been a lost tourist.
"This isn't the first time someone has been found on our premises and shown the door," he told the TT news agency.
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"Sometimes we find visitors who have got lost or have been separated from their group. We're investigating now how the woman got herself in here, and whether she was part of a visiting group, for example."
The police removed the woman from the scene.