Pressing R highlights rail repair units (ie. FBD for Axis) with a yellow outline. However, it also does the same for enemy units. I've only tested this as Axis, so don't know if the same happens with Soviets.
In attached screenshot the units in Krasnodar aren't known, yet one is highlighted as rail repair anyway.
maybe WAD if you already knew of their locations, not WAD if it highlightens formerly unknown units
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