As you note, the Image and Sound Database for the Swiss Social Archives notes the image as being a Zeichen für Blinde/Symbol for the Blind, both here and here.
It appears similar to a Czech symbol apparently indicating disability, as noted in the comments to a facebook post here; one user asks what this parking symbol means:
and another answers that it matches the old symbol for a disabled person:
Google translate has the Czech writing in that image saying "Marking of a vehicle transporting a seriously disabled person" and you can find in the wikipedia page about Czech roadsigns that it dates from at least 1976.
The inverted black triangle itself was a symbol used in Nazi concentration camps to indicate prisoners who were "asozial", including disabled people. I'd hazard a guess that this was the origin for it being used as a general warning sign for disabled people in parts of Europe, but have no actual evidence. The other possibility would be that it developed from the armband with three dots indicating blindness, as you can see on the Czech roadsign page:
According to this page, the yellow armband with three black dots in an inverted triangle formation is used in Germany as a road traffic safety sign indicating blindness or visual impairment, and "was introduced after the First World War, when many war blind people resumed their usual activities and had to be identified by an eye-catching symbol in the road traffic for their own safety." That page goes on to mention that "Today, the stereotypical symbol of the three black dots should be advantageously replaced by a triangle symbol (meanings ▼ for right of way or ▲ for attention) as traffic-safety-sign for all."
Thus, I'd say it's most likely this sign is a warning for drivers to be wary of blind (or possibly any disabled) people crossing the zebra crossing it is placed next to. Note that it actually shares a pole with the more usual zebra crossing sign, as you can see in a long shot in the movie:
