Hi Elisabete,
could be that Excel uses a substitution font for the symbols without letting you know.
To get the Unicode value of the missing glyphs you could select every one and look under Info what the Unicode code point is. Find a font in your repository of fonts that supports the missing glyphs.
What could be helpful is this script by Peter Kahrel that could identify all missing glyphs and applies a character style:
Manage missing glyphs (CS4 and later)
"For years, finding a solution to discovering missing glyphs – the dreaded pink boxes – in InDesign has eluded scripters. Until Peter Baumgartner came up with a solution that works very well. The script on this page uses Peter's solution but adds an interface which shows a document's missing glyphs in a window (rather than in the ESTK), and allows you to apply a font to the missing glyphs."
https://creativepro.com/files/kahrel/indesign/missing_glyphs.html
Regards,
Uwe Laubender
( ACP )