score:15
Here is a virtual tour of a temporary exhibit Magritte La Ligne de vie that was held at MASI Lugano in Switzerland in late 2018 to early 2019. (I found this by way of an Italian language blog post which does not seem to include any details about the painting itself, except for an image.) If you navigate toward the back of the gallery in this virtual your, you can find this, which fits the description perfectly.
Unfortunately it is very difficult to read the sign on the wall in the image above. A reverse Google Image search can lead you to this page, implying that the painting is L’agent secret (The secret agent), 1958 and is held in a private collection. I'm not able to find any more direct verification of this, but the dimensions listed (50 x 70 cm) match perfectly. (The virtual tour allows you to actually measure the painting.)
EDIT: There is a published catalog of the exhibit which probably has more information but is not directly accessible online.
Upvote:8
I think you're referring to a study done for his 1964 painting, 'Le Météore (Étude)' ('The Meteor'). Here's the study. Note that it shows the "lady" facing left rather than right (perhaps the image you remember was shown reversed?). Farther down that page is the final painting, where the sill has been removed and the background changed to a forest.
For bonus points :), the link I included shows that it was sold by Sotheby's in 2016, for $200,000. No word on its present whereabouts. As for "meaning", the linked page includes a short discussion of Magritte's "horse problem" that may be illuminating.