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There is no pre-defined function for finding which contexts a symbol is defined in, but you can use the following to iterate over all known contexts and find the value of a symbol without creating any new symbols in the contexts tested:
valueInAllContexts[symbol_] := Symbol /@ Names[# <> ToString[symbol] & /@ $ContextPath];
var known globally and in two contexts:
{ContextA`var, ContextB`var,var}
valB in, say, one of the contexts ContextB you'd get:
{ContextA`var,valB,var}