Whenever we pursue research and we involve subjects in it, we have to get their consent in order to avoid ethical issues such as putting them in a compromising situation or breaching their privacy.
Not having a secure place to store those forms is not a good excuse, as it is the researcher’s responsibility to ensure their safekeeping.
Similarly, it is the researcher’s duty to provide the subjects with information in their own language and to make sure that they understand the potential risks of the investigation, including potentially discussing embarrassing or intrusive topics.
Finally, a breach of confidentiality is a serious matter, as it can put your participants in an uncomfortable or even dangerous situation, betraying their trust and defeating the whole purpose of obtaining consent.
Therefore, the only time when it is fine to not ask for consent is when there is no record linking the participants to the subject. Their anonymity is automatically protected and a consent document would in fact undermine their privacy.