Roger – it could well be a valuation that the owner/landlord has arranged to assist in the completion of the asset depreciation schedule of their upcoming taxation return or for refinancing, and therefore nothing of concern to you in terms of the unit possibly being listed for sale, but irrespective of the purpose, you would be entirely justified in advising your Managing (rental) Agent – if only to confirm the bonafides of the caller who wants access to your landlord’s property.
That wouldn’t ruffle any feathers, as after all, you’re only acting in the best interests of your owner / landlord 