A glazed panel in the absence of manifestations

From E.L. at The Guardian:

I saw this sign (photo attached) at the Guardian offices in London and, as a frequent (albeit non-linguist) reader of the site, I thought Language Log might be able to assist. I'm genuinely baffled as to its meaning. It may be something to do with being careful about walking into see-through barriers - our building is a very modern steel-and-glass affair, but the big windows are all safely marked with visibility flashes or logos, and there hasn't been a problem in the four years since it opened, as far as I know. The best we could come up with on the subs' desk was that it might mean something like 'Caution: this sign has a glass panel on the front that is hard to see if there is no poster behind it'.

It's a central London office run by a British management company, so it seems unlikely that it was written by someone with English as a second language, but there is an ineffable something of Google Translate about it.

I'm equally baffled. Perhaps a frustrated poet, forced into a career in building management?

so much depends upon a glazed panel

in the absence of manifestations