Now your daughter has been told that she must ask to take medicine. Had she been told that before? Becoming independent and making their own decisions is something kids that age are very good at--sometimes a bit too early.
We're assuming that the cough syrup is OTC; if it is, then it is unlikely to contain ingredients that are dangerous. However, many doctors will prescribe a cough medicine that does contain codeine or similarly dangerous/adictive ingredients. Unlike antibiotics, these are not designed to be all used up at once. So yes, having some in the cabinet is not uncommon.
Without talking to your daughter, you really don't know what is going on, and until you do know, you can't make appropriate decisions. With two bottles of food colouring, yes, you will know, but you won't know whether she had a little or a lot, and that probably matters in terms of why she took it.