Maintaining undescribed means that the object will never be scored equal to or higher than a described object (which includes the compass directions) during adjudication of ambiguous situations where the name of an object is not specified in the command (usually when ‘all’ has been specified or the object name omitted entirely, as in ‘Take’).**
However, when the parser is searching for a single unnamed object and there is no alternative to the undescribed object, it can still be selected- for example in response to ‘Drop’ when the undescribed object is the only item carried.*
Carried things are the biggest problem to keep from the player, since by default they can’t be concealed, can’t be kept out of scope, are stripped of their undescribed status and are always listed in inventory (and, obviously, by definition can’t be moved off-stage). This is all by design- the player is considered to be intimately acquainted with anything they’re directly carrying.
The best strategy if you can’t see a good alternative to a carried-but-completely-hidden object (and there probably is a good alternative) is to:
(i) include the Scopability extension by Brady Garvin and use that to set said item as ‘unscopable’ i.e. definitively removed from scope
(ii) replace the standard ‘carry out taking inventory’ rule to exclude said item from inventory (or to exclude scenery items from inventory and then also make said item portable scenery)
*N.B. ‘For deciding whether all includes’ doesn’t help here, since ‘Drop’ just searches for a single object anyway and even when ‘Drop all’ fails for the dropping action, it falls through to the ‘throwing it at’ action, which again searches for just a single thing to throw and doesn’t invoke ‘For deciding whether all includes’- usually leading to daft responses like ‘(the third eye at the third eye) Futile’ or ‘What do you want to drop the third eye at?’ That grammar could be changed, but the ‘Drop’ issue would still persist.
** N.B.2 Actually, that’s not strictly true. A lower ‘Does the player mean…’ ranking will trump the effect of an object being undescribed during adjudication. As will whether or not the object logically corresponds to the grammar token it’s being matched to (something the parser refers to as the object being ‘good’) e.g. it’s held by the actor for a [something preferably held] token, or it’s a person for a [somebody] token, or in the case of the common [things] token it’s in scope, not undescribed or scenery and not the actor