“Article 38(b) of the Staff Regulations must be interpreted as meaning that it is in the light of the interests of the service that the appointing authority must not only determine the initial period of a secondment based on Article 37, first paragraph, heading (a), of the Staff Regulations, but must also decide, as the case may be, whether to extend that secondment beyond the initial period envisaged. In addition, since the interests of the service are likely to evolve with time, they must be assessed in the light of the circumstances existing at the time when the appointing authority adopts a decision concerning the extension of a secondment…While it was possible for the administration to thus define a time limit for a secondment pursuant to Article 38(b) of the Staff Regulations, the appointing authority could not, however, in the light of its duty to have regard for the welfare of officials and the principle of sound administration, base itself on an assessment carried out at the time when the initial secondment decision was taken in order to release itself from the obligation to carry out a new analysis of the relevant circumstances as they existed on the day on which (a request for a renewal) was examined…In that regard, first, while it is clear from Article 37, first paragraph, heading (a), first indent, of the Staff Regulations that a secondment in the interests of the service is an inherently provisional position, it is also apparent that, under Article 38(b) of those regulations, no time limit is provided for in respect of such a secondment, with the result that those provisions did not in themselves preclude a favourable response to (a request for renewal) for an additional year…(Moreover), it must be observed that, pursuant to Article 38(a) of the Staff Regulations, the decision on secondment in the interests of the service is to be taken by the appointing authority after hearing the official concerned. That provision therefore does not expressly address the question of whether the official concerned has the right to be heard in the case where the appointing authority gives its decision on the renewal of a secondment in the light of a request to that effect made by an EU agency…The appointing authority was under an obligation to provide the applicant with an opportunity effectively to make his views known before it responded to (the Agency)’s request…, irrespective of whether it could have adopted an initial position in that regard, or whether exchanges had previously taken place between the administration and the applicant prior to that request.”