that would be a Unicode "open box" character: ␣ so in Xelatex you can just type it (or insert it using you local Character viewer). Of course you would also need a font that supports it. But there is also an explicit Latex command: https://tex.stackexchange.com/questions/50804/explicit-space-character
– ThrustonJan 07 '19 at 21:38
\verb*|Err: 3.1417 |where the*makes it use visible space. – David Carlisle Jan 07 '19 at 21:42\textvisiblespace. – Bernard Jan 07 '19 at 21:43