Was it an early arrival for the next hunting with dogs vote, or did it have more sinister intent?
Either way, the parliamentary authorities say they are not unduly concerned after a peer reported seeing an urban fox heading for the House of Commons.
Baroness Sharples told the BBC she saw the creature "happily trotting around" outside Parliament.
She has urged people not to feed animals which are "predators".
A parliamentary spokesman said he was not aware of an "immediate problem" and it had a strict pest control policy.
The increasing number of foxes in urban areas has appeared on the political agenda since two young children were attacked by a fox in London earlier this month, suffering serious injuries.
Recent research has suggested there could be as many as 37,000 urban foxes in the country.
Following the incident, the Conservative peer has been pressing ministers to consider measures to "control" the spread of urban foxes.
Baroness Sharples said she had had first-hand experience of the problem, having spotted a fox at the easterly end of the Palace of Westminster as she emerged from the nearby underground station.
"I was rather surprised," she said. "I don't know where it had got in through or where it had come from."
She raised the matter in the House of Lords on Thursday.
Ministers say local councils already have powers to deal with urban foxes but stress people need to take responsibility by being more careful about how they dispose of food waste to reduce opportunities for scavenging.
The peer said some people were still feeding foxes and that "common sense" dictated that, as wild animals, they should not do so.
She told the BBC that she would not be reporting the sighting to the parliamentary authorities, whose director of facilities has ultimate responsibility for the building's pest control policy.
However, she said she was reassured that both houses of Parliament were "very careful" about what they did with their rubbish.
A Lords spokesman said Parliament had suffered mice infestation in the past but there was no "specific" policy for dealing with foxes.